A minister who tried to save the life of the policeman killed in the Westminster terror attack has said "vivid memories" of the atrocity are still with him.

Defence Minister Tobias Ellwood said the hardest part of the day was trying to explain Khalid Masood's deadly attack to his eight-year-old son.

Mr Ellwood, a former soldier, tried in vain to save PC Keith Palmer, giving him mouth-to-mouth resuscitation and applying pressure to try to stem the flow of blood from his wounds after the officer was stabbed outside the Houses of Parliament.

The policeman was one of five people killed by Masood, who hit pedestrians on Westminster Bridge in a hire car before being shot dead by a ministerial bodyguard in March.

Video:April: The Met remembers PC Keith Palmer

Mr Ellwood told The Daily Telegraph: "I think the hardest thing, as well as stepping through with others to try and save PC Keith Palmer's life, was coming home and finding my eight-year-old boy on top of the stairs having refused to go to bed.

"It was 10 o'clock at night and he was really confused. He couldn't understand why a bad person would do what he did and he also couldn't quite understand why I had then stepped forward in the way that I did.

"I had to explain to him that there are some bad people in this world.

"There are bad people doing bad things, but there are more good people doing good things, and that's why we stand up to events such as this."

Video:March: Hero minister addresses MPs

Mr Ellwood had previously been reluctant to talk publicly about what happened, but said he decided to speak out after launching a new mental health strategy for the Armed Forces.

The aim is to encourage veterans to seek help for mental health issues and not to bottle up their experiences.

"What I went through is something... but we shouldn't forget that there are many people who have seen much worse and continue to be affected by it," Mr Ellwood said.

"That's why it is so important for us to have the mental health strategy that we need, a veterans support package that is understood and a covenant that obliges councils, businesses and communities to recognise the sacrifice that individuals have given."